The present invention relates to battery chargers for charging a plurality of rechargeable batteries connected in series. More particularly, this invention relates to battery chargers having a plurality of serially connected battery charging sections. More specifically, although not solely limiting thereto, this invention relates to serial battery chargers in which a battery in any one of the serially connected charging sections can be removed or bypassed without materially affecting the charging conditions of the batteries remaining in other charging sections of the serial battery charger. Furthermore, this invention relates to serial battery chargers in which there is utilized a simple electronic element which provides a low-impedance to the charging circuit during charging, a high-impedance to block reverse current flow from a battery when there is no power supply to the charging section and a comparatively high-impedance when the charging section is shunted or by-passed.
Re-chargeable batteries are widely used in a lot of portable or mobile electrical and electronic devices or appliances such as, cellular or cordless telephones, remote repeaters, remote control units, remote sensors, portable lighting devices, portable radios, portable drills and many other devices. Re-chargeable batteries are generally preferred over disposable batteries nowadays because they are more environmental friendly and provide longer term cost savings. For remote applications, rechargeable batteries are probably the only practical choice.
Re-chargeable batteries require repeated charging in order to supply electrical power to the devices or appliances in which they are installed. Nowadays, portable devices usually require a plurality of batteries to operate and the batteries required are typically in the range of two to ten batteries. Hence, it is desirable that there can be provided intelligent battery chargers which can charge a plurality of re-chargeable batteries at the same time. There are two main types of battery chargers. The first type is the parallel charger in which all the batteries are subject to the same charging voltage but are charged with different charging currents. The other type is the serial charger in which the batteries being charged are connected in series and the same charging current usually passes through all the serially connected batteries.
In applications in which batteries are alternatively charged and discharged, a power supply of 3 to 12 volts is generally required while the voltage of each rechargeable battery is typically in the region of 1-2 volts. In those applications, batteries are typically connected in series for charging and discharging. For charging batteries for use in such applications, a serial battery charger must be used.
Because of the wide-spread use of rechargeable batteries, there are increasing demands for fast battery chargers which are capable of fully charging an empty battery in about an hour (the xe2x80x9c1Cxe2x80x9d chargers) or less time so that users do not have to wait for too long before the batteries are sufficiently charged for use. For example, for a 1,600 mAH re-chargeable battery, the 1C current rate is about 1.6A and the 2C current rate is about 3.2A. In order to facilitate fast and efficient battery charging, battery chargers generally utilise high frequency pulsed charging current having a relatively high current rate. When a battery is being charged, it will produce oxygen on the electrode and the consumption of oxygen by the negative electrode will cause the battery to heat up. In general, charging at the current rate of 1C is preferred because this charging rate is regarded as striking a balance between reducing charging time and maintaining a healthy battery under current battery technologies. Of course, with further advance in battery technologies, batteries may be charged at even higher current ratings without over-heating. If that happens, battery chargers supplying higher charging rating than 1C will be expected to become more popular. In general, fast battery chargers, especially those for charging small voltage re-chargeable batteries of about 1.5-2V, are preferably configured so that the batteries are charged in series. This is because if the batteries are fast charged in parallel, a power supply having a very large current supply rating will be required and this may be very costly.
On the other hand, a serial connection implies that the same current must flow through each serially connected battery charging section. This may also create great difficulty in a lot of circumstances. For example, it may be necessary to remove or isolate a battery from the charger or the charging section upon completion of charging to avoid overheating or damaging or because it is already defective. When a battery is removed from a charging section or the charger, charging will usually be disrupted until a replacement battery has been inserted into the charger. Similar problems also arise if rechargeable batteries of different capacities are charged together or good batteries are mixed with bad ones. This is because when a battery of a smaller capacity has been fully charged, there is a good chance that a battery of a larger capacity still requires charging. For simple serial chargers with no monitoring and control circuits, the batteries will be continuously charged. As a result, overheating, battery damage or even explosion may result. On the other hand, for those more sophisticated serial battery chargers with charging conditions monitoring and charge control circuits, the battery charger may shut down once any one of the batteries being charged is detected as being fully charged. This is obviously undesirable as the remaining batteries may still require further charging. Furthermore, whenever batteries are inserted or removed from a serial battery charger during the charging process, the whole charging process will be interrupted. Hence, it is desirable if there can be provided intelligent serial battery chargers for serial charging of re-chargeable batteries in which the charging currents supplied to the individual batteries in serial connection are largely independent of that supplied to other batteries. In other words, it will be desirable if the charging conditions in a charging section of a serial battery charger comprising a plurality of serially connected charging sections can be substantially independent or isolated from other serial charging sections.
For many battery chargers, it is known that, when power supply to the battery charger is turned off, there may be a reverse leakage current which flows from the battery to the charger or the peripheral circuitry. Reverse leakage current among the serially connected batteries could also cause reverse charging of individual batteries by other batteries that are connected in the series charger. This is clearly an undesirable phenomenon which may cause draining of the full battery capacity and may even damage the charger. Hence, it is desirable that each charging section of a serial battery charger is provided with means to prevent undesirable reverse current leakage as well as a bypassing circuitry so that the charging conditions of one individual charging section would not affect the charging conditions of the other charging sections.
Many bypassing circuits, circuit arrangements or topologies have been proposed to alleviate the adverse influence of the charging conditions in a serial charging section to other charging sections. While serial chargers having arrangements to by-pass some or all of the charging sections have been known, they are generally very complicated and do not simultaneously include means or circuits to prevent reverse leakage or discharge from the batteries.
For example, in U.S. Pat. No. 6,121,752, there is described a battery assembly with a charging current control circuitry which includes a charging current bypassing circuit. The charging current bypassing circuit includes a switching means (50) that is series-coupled with a cell in the battery pack for controlling charging current to the individual cells and a current bypassing means (51) that is parallel-coupled across the switching means and the cell. The switching means and the current bypassing means are switched, i.e. turned ON and OFF, in a complementary fashion. However, in these configurations, when the switching means (50) is turned xe2x80x9cONxe2x80x9d (i.e. closed or actuated), the cell is not prevented from adverse discharge via the switch (50) when an external load or a discharge path is connected. This is particularly problematic when the current bypassing means (51) is also momentarily closed, for example, due to delay or overlap in the control signals, as this may cause short-circuit of the cell through the switches 50 and 51 which may be hazardous. Hence, such configurations are unsatisfactory.
To provide a serial battery charger which fulfils the above requirements is a difficult task because several conflicting requirements need to be met. Firstly, in order to prevent reverse current leakage or adverse current discharge from the battery, a blocking device which has a high reverse impedance must be inserted in series with the battery. Secondly, that serial block device must have a low impedance when there is a forward current which flows into the battery for battery charging. On the other hand, if the blocking device has a low forward impedance when the bypassing switch has been activated (which usually occurs when there is still power supply to the battery charging terminals), that low-impedance blocking device will compete with the bypassing switch for the supplied current and, as a result, adverse charging current will keep flowing into the batteries. In addition, that blocking device must have a high impedance when the bypassing switch has been activated, otherwise, a large and un-desirable current will flow in a current loop which is formed by the battery, the blocking device and the bypassing switch. Hence, it is highly desirable if a serial battery charger which can fulfil the above conflicting requirements can be provided. It will be even more desirable if such improved battery chargers can be realised using simple circuit blocks and components so that high reliability as well as low costs can be achieved.
It is therefore an object of the present invention to alleviate or obviate the problems or shortcomings associated with existing or known serial battery chargers. In particular, it is an object of the present invention to provide circuit arrangements for an improved battery charging section which can be used in serial chargers so that the charging section can be shunted or by-passed when selected and, at the same time, provided with blocking means to prevent reverse current.
An important object of the present invention is therefore to provide an intelligent serial battery charger in which the charging current or charging conditions of one battery or one of the charging sections in the serial connection are largely unaffected by the charging conditions of other batteries or other charging sections in the serial connection. This is particularly important so that a battery can be selectively removed or isolated from the charging section without adversely affecting other charging sections.
Another important object of the present invention is to provide a serial battery charger in which a battery can be removed or isolated from the serially connected battery charging sections without disrupting the charging of other batteries or other charging sections by providing a bypassing shunt and, at the same time, alleviating or avoiding adverse reverse current flow from the battery when the bypassing shunt has been activated.
As a minimum, it is an object of the present invention to provide the public with a choice of serial battery chargers which are provided with means to obviate or alleviate undesirable battery discharge when the battery charger is not supplying charging power and to provide useful battery by-pass as and when necessary.
A battery charger including a plurality of battery charging sections which are connected in series and a charging current source, said charging section includes at least a first branch and a second branch which are connected in parallel, said first parallel branch includes an electronically controllable bypassing switch, said second parallel branch includes terminals for receiving the positive and negative terminals of a battery and a one-way electronic device which are connected in series, said bypassing switch has a very low impedance when activated or turned-on and a very high impedance when deactivated or turned-off, wherein said one-way electronic device is characterised by a very low-impedance when current flows from said charging section into said battery terminals and a high-impedance when said bypassing switch is activated, said one-way electronic device allows charging current to flow into said battery but substantially prevents discharge of said battery through said one-way electronic device.
According to another aspect of the present invention, there is provided a serial battery charger including a battery charging section which includes at least first and second parallelly connected branches, wherein said first branch includes a diode serially connected with the terminals for connecting the battery to be charged and said second branch includes a MOSFET bypassing switch, said bypassing switch is connected across said first branch and provides low-impedance shunting when activated, said blocking diode has a low-impedance when current flows into said battery to be charged and has a high-impedance when there is no power supply from said battery charger or when said bypassing switch is turned on.
Preferably, said bypassing switches of said plurality of battery charging sections can be selectively activated and deactivated.
Preferably, said charging current source includes a constant current source and said charger further includes a micro-controller, said bypassing switches of said plurality of charging sections being selectively activatable by said micro-controller.
Preferably, said one-way electronic behaves as a current blocking device which substantially blocks current flowing in or out of a battery when the bypassing switch of that said charging section containing that said battery has been activated.
Preferably, the batteries in said plurality of charging sections can be selectively isolated from the charging sections by selective activation of said bypassing switches.
Preferably, said selectively isolated batteries are also isolated from batteries in other charging sections such that the isolated batteries will not significantly interfere electrically with the batteries in other charging sections.
Preferably, said charger including means for measuring the open-circuit electrical parameters of a battery in a charging section when the bypassing switch of that charging section has been selectively activated.
Preferably, said charger further including means for measuring the characteristic electrical parameters of a battery in a charging section by selective activation of the bypassing switches.
Preferably, the bypassing switch of a said charging section is connected in parallel with the serial connection of said battery terminals and said one-way electronic device of that charging section.
Preferably, the activation states of said bypassing switch and said one-way electronic device of the same charging section being opposite.
According to a preferred embodiment, said one-way electronic device includes a diode.
According to a preferred embodiment, said bypassing switch includes a three-terminal device in which the impedance across two of its terminals is controllable by a third terminal.
Preferably, said two of the terminals of said three-terminal device of a charging section are connected in parallel across the serial connection of said battery terminals and said one-way electronic device of the charging section.
According to a preferred embodiment, said three-terminal device includes a MOSFET with a relatively high switching bandwidth, the drain and source terminals of said MOSFET being connected in parallel with the serial connection of the battery terminals and the one-way electronic device of the charging section.
Preferably, said three-terminal device is a device having a relatively high switching bandwidth.
Preferably, said one-way electronic device includes a blocking device having a considerably higher impedance than that of the activated bypassing switch when said bypassing switch is turned on.
Preferably, said one-way electronic devices can be selectively activated or deactivated.
Preferably, said one-way electronic device includes a three-terminal device in which the impedance across two of its terminals is controllable by a third terminal.
Preferably, said third terminal of said three-terminal one-way electronic device being connected to the switching control terminal of said bypassing switch via an logic inversion means such as an invertor or a NAND gate.
Preferably, either said third terminal of said one-way electronic device and said third terminal of said bypassing switching is controllable by the same control port of a microcontroller.
Preferably, a logic inversion means is connected between said third terminals of said one-way electronic device and said bypassing switch.
According to a preferred embodiment, said one-way electronic device includes a Metal Oxide-Semiconductor Field Effect Transistor (MOSFET), said third terminals of said one-way electronic devices being the gate terminal.
Preferably, said one-way electronic device includes a diode, a MOSFET or like device.
Preferably, the battery in the charging section in which said bypassing switch is activated is substantially isolated electrically from the charging current, the other charging sections and the other batteries by said one-way electronic device.